An exchange spring magnet behaves as a single hard magnetic material because of strong exchange bonding force between both phases described above and, at the same time, it exhibits such a specific behavior that magnetization reversibly springs back to a change of an external magnetic field in the second quadrant of a demagnetization curve. Recently, an optimum use of the effect has attracted special interest.
A suggested method of allowing a soft magnetic phase to exist in an alloy for magnet is roughly divided into two ways. The method belonging to the first division is a method of causing the soft magnetic phase separation as a result of p precipitation from a molten alloy with a controlled composition on solidification during cooling or the following heat treatment after cooling, and includes various methods, for example, a method described in Unexamined Patent Publication No. Hei 5-135928 wherein a Nd--Fe--B alloy containing excess Fe is molten, solidified and heat-treated to obtain a micro-crystal aggregate of a Fe.sub.3 B phase (soft magnetic phase) and a Nd.sub.2 Fe.sub.14 B phase (hard magnetic layer), or a method described in Unexamined Patent Publication No. Hei 6-330252 wherein a Sm--Fe--N alloy containing excess Fe is molten, solidified and heat-treated, thereby allowing to make a Fe phase (soft magnetic phase) and a Sm.sub.2 Fe.sub.17 N.sub.x phase (hard magnetic layer) coexist as a crystal having a grain size of not more than 0.5 .mu.m, respectively. However, these alloys obtained by these methods can be used only as an isotropic magnet alloy and have such disadvantages that there is a limitation in characteristics for novel use in future and that expensive and large-scale equipment for melting and quenching solidification of the alloy are required.
The method belonging to the second division is a method of using needle-like iron powder as a base material and changing the surface portion into a hard magnetic phase by using a chemical treatment and a heat treatment. Unexamined Patent Publication No. Hei 7-272913 discloses a raw material for permanent magnet, comprising needle-like iron powder, an aluminum phosphate coating layer, a rare earth diffusion layer or a rare earth-iron-boron diffusion layer or a rare earth-boron-nitrogen diffusion layer, and an aluminum phosphate coating layer, said layers being provided in order on the surface of the needle-like iron powder, and also discloses a method for producing the raw material, which comprises the steps of heating FeOOH (Goethite) needle-like grains under the state of being coated with aluminum phosphate in a hydrogen atmosphere to 300-500.degree. C., thereby reducing FeOOH to Fe (needle-like iron powder); heating to 650-1000.degree. C. in an argon atmosphere under the presence of rare earth or rare earth and boron, thereby diffusing rare earth, or rare earth and boron on the surface of the aluminum phosphate-coated needle-like iron powder; heating to 500-300.degree. C. in a nitrogen atmosphere, thereby diffusing nitrogen on the surface layer; and heating to 300-500.degree. C in an argon atmosphere, thereby coating with aluminum phosphate again. According to this method, magnetic characteristics are improved by the oxidizing inhibition effect due to double coating of aluminum phosphate and the action as a magnetic domain wall thereof, but stable excellent magnetic characteristics can not be obtained. This reason is as follows. That is, during the evaporation and diffusion of Sm, aluminum phosphate is decomposed and reduced by a strong reducing force of Sm and Al is incorporated into the iron powder, whereas, Sm is oxidized and the hard magnetic phase of the Sm--Fe--N alloy is not easily formed, resulting in deterioration of magnetic characteristics.
More particularly, the present invention relates to an improvement in exchange spring magnet by using the method belonging to the second division, and an object of the present invention is to provide a powder for permanent magnet having stable excellent magnetic characteristics by homogeneously diffusing and forming a hard magnetic layer on the surface of needle-like Fe fine particles, a method for producing the powder, and an anisotropic permanent magnet made by the powder.